A method of fabricating a GaN substrate using a sapphire underlying substrate has been conventionally proposed. The conventional GaN substrate fabrication method will be explained below with reference to FIGS. 1 to 3. FIGS. 1 to 3 are sectional views showing the steps in the GaN substrate fabrication method.
In a step shown in FIG. 1, a sapphire underlying substrate 110 is prepared as an underlying substrate. A peeling layer (low-temperature GaN buffer layer) 120 is grown on the sapphire underlying substrate 110 at a temperature (low temperature) lower than 1,000° C. The peeling layer 120 can be made of, e.g., a single-crystal material, polycrystalline material, or amorphous material of GaN. Then, a GaN layer 130 is grown at a temperature (high temperature) of about 1,000° C. The GaN layer 130 can be made of, e.g., a single-crystal material of GaN. In this manner, a structure 100 including the peeling layer 120 and GaN layer 130 is formed on the sapphire underlying substrate 110. Note that the peeling layer 120 also has a buffering function.
In a step shown in FIG. 2, the sapphire underlying substrate 110 and structure 100 (in a reaction chamber) are cooled from about 1,000° C. to room temperature. The thermal expansion coefficient of the sapphire underlying substrate 110 is higher than that of the GaN layer 130. When the temperature is lowered from about 1,000° C. to room temperature, therefore, a thermal stress resulting from the thermal expansion coefficient difference acts on the sapphire underlying substrate 110 and GaN layer 130, and warping occurs.
In a step shown in FIG. 3, the peeling layer 120 is melted by the laser lift-off method or the like. This separates the GaN layer 130 from the sapphire underlying substrate 110. That is, a GaN substrate is fabricated by separating the GaN layer 130 from the sapphire underlying substrate 110. In this state, the GaN layer 130 produces a portion in which the internal stress has reduced, and a portion in which the internal stress remains. This may crack the GaN layer 130.
The separated GaN layer 130 warps because the distribution of strain changes along the crystal growth direction. Accordingly, even when the GaN layer is planarized by a mechanical polishing step, the crystal orientation is distorted as shown in FIG. 3.
To reduce this warping, a method of forming a gap between the sapphire underlying substrate 110 and peeling layer 120 has been proposed (e.g., patent reference 1).    Patent Reference 1: Japanese Patent Laid-Open No. 2004-39810